Rolled magnesium alloy material, magnesium alloy structural member, and method for producing rolled magnesium alloy material

ABSTRACT

Provided are a rolled Mg alloy material which has a wide width and whose mechanical properties are uniform in a width direction, a Mg alloy structural member produced by plastically working the rolled Mg alloy material, and a method for producing the rolled Mg alloy material. The method for producing a rolled Mg alloy material includes rolling a Mg alloy material with a reduction roll. The Mg alloy material has a width of 1,000 mm or more, and the reduction roll has three or more regions in the width direction. The temperature is controlled in each of the regions so that a difference between a maximum temperature and a minimum temperature is 10° C. or less in the width direction of a surface of the reduction roll. The variation in the rolled state in the width direction can be reduced by reducing a difference in temperature over the width direction of the reduction roll. As a result, it is possible to produce a rolled Mg alloy material whose mechanical properties are substantially uniform in the width direction.

TECHNICAL FIELD

The present invention relates to a rolled magnesium alloy material, a magnesium alloy structural member, and a method for producing a rolled magnesium alloy material. In particular, the present invention relates to a rolled magnesium alloy material that exhibits uniform mechanical properties in a width direction even when the rolled material has a wide width, a magnesium alloy structural member obtained by plastically working the rolled magnesium alloy material, and a method for producing the rolled magnesium alloy material.

BACKGROUND ART

Recently, a magnesium (hereinafter, Mg) alloy sheet has been used in, for example, housings of cellular phones and laptop computers. Since Mg alloys have poor plastic workability, cast materials produced by die casting or thixomolding are mainly used. In general, such cast materials are subjected to, for example, rolling so as to improve the mechanical properties thereof.

PTL 1 describes that rolling is performed on a cast material composed of a magnesium alloy corresponding to the AZ91 alloy in the American Society for Testing and Materials (ASTM) standards, the cast material being produced by a twin-roll continuous casting process. Specifically, the rolling is performed while respectively controlling a surface temperature of a Mg alloy material sheet immediately before the sheet is inserted into reduction rolls and a surface temperature of the reduction rolls to specific temperatures.

CITATION LIST Patent Literature

-   PTL 1: Japanese Unexamined Patent Application Publication No.     2007-098470

SUMMARY OF INVENTION Technical Problem

With expansion of the range of applications of Mg alloys, the development of Mg alloy materials having large dimensions has been desired. In the rolling described above, for example, in the case where a Mg alloy material has a relatively narrow width, a surface temperature of the Mg alloy material and a surface temperature of the reduction rolls naturally easily become uniform in the width direction. Therefore, the rolled state does not tend to vary in the width direction, and a rolled Mg alloy material whose mechanical properties are uniform in the width direction is easily produced. However, with an increase in the width of the alloy material, in particular, when the width is 1,000 mm or more, it is difficult to make the mechanical properties uniform in the width direction. The reason for this is as follows. As the width increases, during rolling, the heated state tends to be more easily maintained in a central portion in the width direction of the Mg alloy material, and the Mg alloy material tends to more easily cool in both edge portions in the width direction. Accordingly, it is difficult to maintain a uniform heated state in the central portion and the edge portions, which may result in the difference in the rolled state.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a rolled Mg alloy material which has a wide width and whose mechanical properties are uniform in a width direction.

Another object of the present invention is to provide a Mg alloy structural member using the rolled Mg alloy material.

Another object of the present invention is to provide a method for producing the rolled Mg alloy material.

Solution to Problem

A rolled Mg alloy material of the present invention is produced by rolling a Mg alloy material with a reduction roll, and the rolled Mg alloy material has a width of 1,000 mm or more. In a width direction of the rolled Mg alloy material, a ratio O_(E)/O_(C) of a basal plane peak ratio of an edge portion to a basal plane peak ratio of a central portion satisfies 0.89≦O_(E)/O_(C)≦1.15, where the basal plane peak ratio O_(C) of the central portion and the basal plane peak ratio O_(E) of the edge portion are respectively represented by formulae below:

Basal plane peak ratio O_(C): I_(C)(002)/{I_(C)(100)+I_(C)(002)+I_(C)(101)+I_(C)(102)+I_(C)(110)+I_(C)(103)}

Basal plane peak ratio O_(E): I_(E)(002)/{I_(E)(100)+I_(E)(002)+I_(E)(101)+I_(E)(102)+I_(E)(110)+I_(E)(103)}

In the formulae, I_(C)(002), I_(C)(100), I_(C)(101), I_(C)(102), I_(C)(110), and I_(C)(103) respectively represent X-ray diffraction peak intensities of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane, and a (103) plane in the central portion in the width direction of the rolled Mg alloy material, and I_(E)(002), I_(E)(100), I_(E)(101), I_(E)(102), I_(E)(110), and I_(E)(103) respectively represent X-ray diffraction peak intensities of the (002) plane, the (100) plane, the (101) plane, the (102) plane, the (110) plane, and the (103) plane in the edge portion in the width direction.

According to the rolled Mg alloy material of the present invention, since the ratio O_(E)/O_(C) of the basal plane peak ratio of the edge portion to the basal plane peak ratio of the central portion of the rolled Mg alloy material satisfies the above range, the orientation of the crystal plane is uniform in the width direction. Thus, it is possible to provide a rolled Mg alloy material whose plastic workability (formability) is uniform in the width direction. Accordingly, when plastic working is performed on this rolled material, substantially uniform working can be performed regardless of the position of the rolled material.

In the rolled material according to an embodiment of the present invention, an average grain size ratio D_(E)/D_(C) of the edge portion to the central portion may satisfy 0.7≦D_(E)/D_(C)≦1.5, where D_(C) denotes an average grain size of the central portion of a cross section orthogonal to a rolling direction and D_(E) denotes an average grain size of the edge portion of a cross section orthogonal to a rolling direction.

In this case, since the average grain size ratio D_(E)/D_(C) of the average grain size of the edge portion to the average grain size of the central portion of the rolled Mg alloy material satisfies the above range, the average grain size is uniform in the width direction. Accordingly, it is possible to provide a rolled material whose strength and corrosion resistance are uniform in the width direction.

In the rolled material according to an embodiment of the present invention, an elongation ratio E_(E)/E_(C) of the edge portion to the central portion may satisfy 2/3≦E_(E)/E_(C)≦3/2, where E_(C) denotes an elongation of the central portion in a tensile test in a rolling direction and E_(E) denotes an elongation of the edge portion in a tensile test in a rolling direction.

In this case, since the elongation ratio E_(E)/E_(C) of the elongation of the edge portion to the elongation of the central portion of the rolled Mg alloy material satisfies the above range, the elongation in the rolling direction is uniform in the width direction. In other words, substantially uniform plastic working can be performed regardless of the position of the rolled material.

In the rolled material according to an embodiment of the present invention, a tensile strength ratio Ts_(E)/Ts_(C) of the edge portion to the central portion may satisfy 0.9≦Ts_(E)/Ts_(C)≦1.1, where Ts_(C) denotes a tensile strength of the central portion in a tensile test in a rolling direction and Ts_(E) denotes a tensile strength of the edge portion in a tensile test in a rolling direction.

In this case, since the tensile strength ratio Ts_(E)/Ts_(C) of the tensile strength of the edge portion to the tensile strength of the central portion of the rolled Mg alloy material satisfies the above range, the tensile strength in the rolling direction is substantially uniform in the width direction.

In the rolled material according to an embodiment of the present invention, a 0.2% proof stress ratio Ps_(E)/Ps_(C) of the edge portion to the central portion may satisfy 0.9≦Ps_(E)/Ps_(C)≦1.1, where Ps_(C) denotes a 0.2% proof stress of the central portion in a tensile test in a rolling direction and Ps_(E) denotes a 0.2% proof stress of the edge portion in a tensile test in a rolling direction.

In this case, since the 0.2% proof stress ratio Ps_(E)/Ps_(C) of the 0.2% proof stress of the edge portion to the 0.2% proof stress of the central portion of the rolled Mg alloy material satisfies the above range, the proof stress ratio in the rolling direction is uniform in the width direction. Accordingly, it is possible to provide a rolled material whose formability in the rolling direction is substantially uniform in the width direction.

In the rolled material according to an embodiment of the present invention, the magnesium alloy material may contain aluminum in an amount of 5% by mass or more and 12% by mass or less.

In this case, since the Mg alloy material contains aluminum in an amount in the range mentioned above, a rolled Mg alloy material having a higher hardness and excellent corrosion resistance can be provided.

A Mg alloy structural member of the present invention is produced by plastically working the rolled Mg alloy material of the present invention.

In this case, since plastic working is performed on a rolled Mg alloy material whose mechanical properties are uniform in the width direction, it is possible to provide a Mg alloy structural member having uniform properties regardless of the position thereof.

A method for producing a rolled Mg alloy material of the present invention includes rolling a magnesium alloy material with a reduction roll. The magnesium alloy material has a width of 1,000 mm or more, and the reduction roll has three or more regions in a width direction. In addition, the temperature is controlled in each of the regions so that a difference between a maximum temperature and a minimum temperature is 10° C. or less in the width direction of a surface of the reduction roll.

According to the production method of the present invention, by reducing the difference in temperature of reduction rolls over the width direction, the variation in the rolled state in the width direction can be reduced. Therefore, rolling that is uniform in the width direction can be performed on a Mg alloy material having a wide width of 1,000 mm or more. Accordingly, it is possible to produce a rolled Mg alloy material in which the variation in the thickness, edge cracks, etc. are suppressed, which has a width of 1,000 mm or more, and whose mechanical properties are substantially uniform in the width direction.

In the production method according to an embodiment of the present invention, the temperature may be controlled by introducing, into the reduction roll, heat transfer oil whose temperature has been adjusted.

In this case, since the temperature is controlled by using heat transfer oil, the temperature can be rapidly controlled to a predetermined temperature in each of the regions from the inside of the reduction rolls.

In the production method according to an embodiment of the present invention, the temperature may be controlled by allowing a heating fluid whose temperature has been adjusted to adhere to the surface of the reduction roll.

In this case, since the temperature is controlled by allowing a heating fluid to directly adhere to the surfaces of the rolls, the temperature can be finely controlled in the width direction of the reduction rolls, for example, in each of the regions and in a portion that extends over adjacent regions. In addition, a temperature control mechanism need not be installed inside the reduction rolls. That is, even in existing reduction rolls that do not include a temperature control mechanism, the surface temperature of the reduction rolls can be easily controlled in each region from the outside of the rolls by using the heating fluid.

In the production method according to an embodiment of the present invention, the temperature may be controlled so that, on the surface of the reduction roll, a difference in temperature between two points 100 mm away from each other in the width direction is 6° C. or less.

In this case, by reducing the difference in temperature between two close points, the variation in the temperature distribution of the reduction rolls can be easily controlled over the width direction. Thus, the variation in the rolled state can be reduced in the width direction of the Mg alloy material.

In the production method according to an embodiment of the present invention, preheating may be performed so that, on a surface of the magnesium alloy material immediately before the magnesium alloy material passes through the reduction roll, a difference between a maximum temperature and a minimum temperature in the width direction is 8° C. or less.

In this case, by reducing the difference in temperature of the Mg alloy material over the width direction, the variation in the rolled state can be more effectively reduced in the width direction of the Mg alloy material. Specifically, rolling can be uniformly performed by reducing the variation not only in the temperature of the reduction rolls but also in the temperature of the material in the width direction.

In the production method according to an embodiment of the present invention, preheating may be performed so that, on a surface of the magnesium alloy material immediately before the magnesium alloy material passes through the reduction roll, a difference in temperature between two points 100 mm away from each other in the width direction is 6° C. or less, and the temperature may be controlled so that, on the surface of the rolled magnesium alloy material immediately after the magnesium alloy material passes through the reduction roll, a difference in temperature between two points 100 mm away from each other in the width direction is 6° C. or less.

In this case, the variation in the temperature distribution of the reduction rolls can be more easily controlled over the width direction by reducing the difference in temperature between two close points before and after rolling. Thus, the variation in the rolled state can be more effectively reduced in the width direction of the Mg alloy material.

Advantageous Effects of Invention

A rolled Mg alloy material of the present invention has a wide width and mechanical properties that are uniform in the width direction.

According to a Mg alloy structural member of the present invention, the properties can be made uniform at any position.

According to a method for producing a rolled Mg alloy material of the present invention, even when a Mg alloy material has a wide width of 1,000 mm or more, a rolled material whose mechanical properties are uniform in the width direction can be produced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 includes schematic views of a process for producing a rolled Mg alloy material according to an embodiment, part (A) is a view that schematically illustrates an example of a rolling line, and part (B) is a view that illustrates a heat box used for preheating a Mg alloy material.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will now be described. First, a rolled Mg alloy material will be described, and subsequently, a method for producing the rolled Mg alloy material will be described with reference to FIG. 1, as required.

<<Rolled Mg Alloy Material>>

[Composition]

Examples of a rolled Mg alloy material include materials having various compositions containing Mg as a main component, and additive elements added to the Mg (balance: inevitable impurities). In particular, in the present invention, Mg—Al alloys containing at least aluminum (Al) as an additive element are preferable. With an increase in the Al content, not only corrosion resistance tends to be high but also mechanical properties such as a strength and plastic deformation resistance tend to be high. Accordingly, in the present invention, Al is preferably contained in an amount of 3% by mass or more, 5% by mass or more, particularly preferably 7.0% by mass or more, and still more preferably 7.3% by mass or more. However, an Al content exceeding 12% by mass decreases plastic workability, and thus the upper limit of the Al content is 12% by mass. The Al content is particularly preferably 11% by mass or less, and still more preferably 8.3% to 9.5% by mass.

The additive elements other than Al may be at least one selected from zinc (Zn), manganese (Mn), silicon (Si), beryllium (Be), calcium (Ca), strontium (Sr), yttrium (Y), copper (Cu), silver (Ag), tin (Sn), nickel (Ni), gold (Au), lithium (Li), zirconium (Zr), cerium (Ce), and rare earth elements RE (excluding Y and Ce). In the case where these elements are contained, the content thereof is, for example, 0.01% by mass or more and 10% by mass or less in total, and preferably 0.1% by mass or more and 5% by mass or less in total. When, among these additive elements, at least one element selected from Si, Sn, Y, Ce, Ca, and rare earth elements (excluding Y and Ce) is contained in an amount of 0.001% by mass or more, and preferably 0.1% by mass or more and 5% by mass or less in total, good heat resistance and good flame retardancy are obtained. When rare earth elements are contained, the total content thereof is preferably 0.1% by mass or more. In particular, when Y is contained, the content thereof is preferably 0.5% by mass or more. An example of the impurities is Fe.

Examples of the specific compositions of the Mg—Al alloys include AZ alloys (Mg—Al—Zn alloys, Zn: 0.2% to 1.5% by mass), AM alloys (Mg—Al—Mn alloys Mn: 0.15% to 0.5% by mass), Mg—Al-RE (rare earth element) alloys, AX alloys (Mg—Al—Ca alloys, Ca: 0.2% to 6.0% by mass), and AJ alloys (Mg—Al—Sr alloys, Sr: 0.2% to 7.0% by mass) in the ASTM standards. In particular, a Mg—Al alloy containing 8.3% to 9.5% by mass of Al and 0.5% to 1.5% by mass of Zn, typically, the AZ91 alloy is preferable in view of good corrosion resistance and mechanical properties.

[Dimensions]

The length and the thickness of the rolled Mg alloy material may be appropriately selected in accordance with the size of a magnesium alloy structural member to be produced, and are not particularly limited as long as the width of the rolled Mg alloy material is 1,000 mm or more. Examples of the rolled Mg alloy material include long materials and short materials produced by cutting a coil material to have an appropriate length. Regardless of the length of the rolled material, the rolled material preferably has a thickness that is substantially uniform in the width direction. In particular, a thickness ratio t_(E)/t_(C) preferably satisfies 0.97≦t_(E)/t_(C)≦1.03 where t_(C) denotes a thickness of a central portion in the width direction of a rolled Mg alloy material and t_(E) denotes a thickness of an edge portion in the width direction of the rolled Mg alloy material. When this range is satisfied, the thickness of the rolled Mg alloy material is uniform in the width direction. Accordingly, when the rolled Mg alloy material is wound as a coil, the occurrence of winding deviation can be suppressed. Herein, the term “central portion” refers to a range extending from the center in the width direction of a rolled material to positions spaced away from the center by about 50 mm or less in directions toward the edges on both sides. The term “edge portion” refers to a range extending from a side edge to near a position about 100 mm or less, and preferably about 50 mm or less from the side edge in a direction toward the center. Hereinafter, the terms “central portion” and “edge portion” respectively refer to the same positions as the central portion and the edge portion defined above.

[Mechanical Properties]

According to the rolled Mg alloy material of the present invention, even when the rolled Mg alloy material has a width of 1,000 mm or more, physical values described below can be made uniform over the width direction by making the rolled state uniform in the width direction as described below. Specific mechanical properties will be described below.

(Basal Plane Peak Ratio)

A basal plane peak ratio is determined by X-ray diffractometry with respect to a central portion and an edge portion in the width direction of a rolled Mg alloy material. Herein, a basal plane peak ratio O_(C) in the central portion is represented by I_(C)(002)/{I_(C)(100)+I_(C)(002)+I_(C)(101)+I_(C)(102)+I_(C)(110)+I_(C)(103)} on the basis of peak intensities I_(C)(002), I_(C)(100), I_(C)(101), I_(C)(102), I_(C)(110), and I_(C)(103) determined by X-ray diffraction of the (002) plane, the (100) plane, the (101) plane, the (102) plane, the (110) plane, and the (103) plane, respectively. Similarly, a basal plane peak ratio O_(E) in the edge portion is represented by I_(E)(002)/{I_(E)(100)+I_(E)(002)+I_(E)(101)+I_(E)(102)+I_(E)(110)+I_(E)(103)} on the basis of peak intensities I_(E)(002), I_(E)(100), I_(E)(101), I_(E)(102), I_(E)(110), and I_(E)(103) determined by X-ray diffraction of the (002) plane, the (100) plane, the (101) plane, the (102) plane, the (110) plane, and the (103) plane, respectively. When a ratio O_(E)/O_(C) of the basal plane peak ratio of the edge portion to the basal plane peak ratio of the central portion determined as described above satisfies 0.89≦O_(E)/O_(C)≦1.15, it is determined that the basal plane peak ratio is substantially uniform in the width direction. Such a rolled Mg alloy material has a uniform orientation of the crystal plane in the width direction of the rolled Mg alloy material, and can have substantially uniform plastic workability (formability) in the width direction. Regarding the positions measured by X-ray diffractometry, the measurement is performed on a surface in each of the central portion and the edge portion.

(Average Grain Size)

In each of the central portion and the edge portion, an average grain size on a cross section orthogonal to a rolling direction is determined in accordance with “Steels-Micrographic determination of the grain size JIS G 0551 (2005)”. When an average grain size ratio D_(E)/D_(C) satisfies 0.7≦D_(E)/D_(C)≦1.5 where D_(E) denotes the average grain size of the edge portion and D_(C) denotes the average grain size of the central portion, it is determined that the average grain size is substantially uniform in the width direction. In such a rolled Mg alloy material, the strength and corrosion resistance can be made substantially uniform in the width direction. The average grain size ratio D_(E)/D_(C) is more preferably 0.9≦D_(E)/D_(C)≦1.1.

(Elongation·Tensile Strength·0.2% Proof Stress)

An elongation, a tensile strength, and a 0.2% proof stress are determined in each of the central portion and the edge portion in accordance with “Method of tensile test for metallic materials JIS Z 2241 (1998)”. In each of the central portion and the edge portion, a JIS No. 13B specimen (JIS Z 2201 (1998)) is cut so that the longitudinal direction of the specimen corresponds to the rolling direction, and the tensile test is performed using the specimen.

When an elongation ratio E_(E)/E_(C) satisfies 2/3≦E_(E)/E_(C)≦3/2 where E_(E) denotes the elongation of the edge portion and E_(C) denotes the elongation of the central portion, it is determined that the elongation is substantially uniform in the width direction.

Similarly, when a tensile strength ratio Ts_(E)/Ts_(C) satisfies 0.9≦Ts_(E)/Ts_(C)≦1.1 where Ts_(E) denotes the tensile strength of the edge portion and Ts_(C) denotes the tensile strength of the central portion, it is determined that the tensile strength is substantially uniform in the width direction.

When a 0.2% proof stress ratio Ps_(E)/Ps_(C) satisfies 0.9≦Ps_(E)/Ps_(C)≦1.1 where Ps_(E) denotes the 0.2% proof stress of the edge portion and Ps_(C) denotes the 0.2% proof stress of the central portion, it is determined that the 0.2% proof stress is substantially uniform in the width direction.

When the elongation, the tensile strength, and the 0.2% proof stress satisfy the above ranges, the formability can be made uniform in the width direction.

<Magnesium Alloy Structural Member>

A Mg alloy structural member is obtained by plastically working the rolled Mg alloy material of the present invention. Various types of working such as press working, deep-drawing, forging, and bending can be employed as the plastic working. Examples of the plastically worked Mg alloy structural member typically include structural members, the whole of which are subjected to plastic working, such as plastically worked structural members each having a three-dimensional shape, e.g., tubular structural members and wave-shaped structural members, and structural members produced by plastically working a part of a rolled Mg alloy material, i.e., structural members having a portion that has been subjected to plastic working. Since the rolled Mg alloy material of the present invention has mechanical properties that are uniform in the width direction, a position to be subjected to plastic working can be appropriately selected without limitation. Therefore, plastic working such as bending can be freely performed. The plastic working may be performed while the rolled material is heated at 200° C. to 300° C. In such a case, breaking etc. are not readily generated, and thus a Mg alloy structural member having a good surface texture is obtained. Furthermore, since the rolled Mg alloy material has mechanical properties that are uniform in the width direction as described above, the resulting Mg alloy structural member has uniform properties at any position.

Alternatively, the rolled Mg alloy material of the present invention may be subjected to working for changing the shape thereof, such as cutting and punching, as required. Thus, a sheet-like Mg alloy structural member having a predetermined shape can be obtained.

The resulting Mg alloy structural member may be subjected to a surface texture-modifying treatment such as polishing, an anti-corrosion treatment such as a chemical conversion treatment or an anodization treatment, or a decorative surface treatment such as painting, thereby further improving corrosion resistance, providing mechanical protection, and enhancing the commercial value.

<<Method for Producing Rolled Mg Alloy Material>>

The above-described rolled Mg alloy material which has a width of 1,000 mm or more and whose mechanical properties are uniform in the width direction is produced by rolling a Mg alloy material with reduction rolls. This rolling is performed as follows: As illustrated in FIG. 1(A), a Mg alloy material sheet 1 unwound from a reel 10 a (10 b) is rolled with reduction rolls 3, and the rolled material sheet 1 is taken up on another reel 10 b (10 a). This operation is defined as one pass, and the operation is performed for a plurality of passes. In this embodiment, reverse rolling is performed in which the rotation direction of each reel 10 a (10 b) is reversed for every pass. A protective cover 5 is arranged between the reel 10 a (10 b) and the reduction rolls 3 so that a surface temperature of the Mg alloy material sheet 1 does not decrease on the upstream and downstream of the reduction rolls. Temperature sensors 4 r, 4 bf, and 4 bb are provided. The temperature sensors 4 r, 4 bf, and 4 bb respectively measure a surface temperature of the reduction rolls 3, a surface temperature of the material sheet 1 immediately before the material sheet 1 passes through the reduction rolls 3, and a surface temperature of the material sheet 1 immediately after the material sheet 1 passes through the reduction rolls 3. A feature of the production method of the present invention is that each of the reduction rolls has three or more regions in the width direction, and the temperature is controlled in each of the regions so that a difference between the maximum temperature and the minimum temperature in the width direction of a surface of the reduction roll is 10° C. or less, whereby the rolled Mg alloy material of the present invention can be obtained. This method will now be described in more detail.

[Preparation of Mg Alloy Material]

(Casting)

First, a Mg alloy material sheet 1 is prepared. A cast material (cast sheet) having the same composition as the composition of the rolled material described above can be suitably used as the Mg alloy material sheet 1. The cast material is produced by a continuous casting process, such as a twin-roll casting process, or die casting. In particular, since rapid solidification can be performed by the twin-roll casting process, internal defects such as oxides and segregated products can be reduced and it is possible to suppress the generation of breaking etc. originated from the internal defects during plastic working such as rolling. That is, the twin-roll casting process is preferable from the standpoint of producing a cast material having a good rolling property. In particular, in a Mg alloy material having a large Al content, impurities in crystal and precipitated impurities, and segregated products are easily generated during casting, and such impurities in crystal and precipitated impurities, and segregated products tend to remain in the material even after a process such as rolling is performed after casting. However, as described above, segregation etc. can be suppressed in a cast material produced by the twin-roll casting process, and thus such a cast material can be suitably used as a Mg alloy material. The thickness of the cast material is not particularly limited. However, when the thickness of the cast material is excessively large, segregation tends to occur. Accordingly, the thickness is preferably 10 mm or less, more preferably 5 mm or less, and particularly preferably 4 mm or less. The width of the cast material is 1,000 mm or more. A cast material having a width that can be produced with production equipment can be used. In this embodiment, a long cast material produced by casting is wound in the form of a coil to prepare a cast coil material, and the cast coil material is used in the subsequent step. During winding, in particular, the temperature of a winding start portion of the cast material may be about 100° C. to 200° C. In such a case, even an alloy in which breaking readily occurs, such as the AZ91 alloy, is easily bent and easily wound.

(Solution Treatment)

Rolling may be performed on the cast material. Alternatively, a solution treatment may be performed on the cast material before rolling, and the solution-treated material may be used as the Mg alloy material sheet 1. The cast material can be homogenized by the solution treatment. For example, the conditions for the solution treatment are as follows. The holding temperature is 350° C. or higher, and preferably 380° C. to 420° C., and the holding time is 30 to 2,400 minutes. With an increase in the Al content, it is preferable to increase the holding time. In a cooling step after the holding time, the cooling rate may be increased by using, for example, forced cooling such as water cooling or air blast. In this case, precipitation of coarse precipitates can be suppressed to produce a sheet having a good rolling property. In the case where a solution treatment is performed on a long cast material, the cast material may be wound in the form of a coil and the solution treatment may then be performed in this state, as in the cast coil material. In this case, the long cast material can be efficiently heated.

[Preheating]

The cast material or the Mg alloy material that has been subjected to a solution treatment is rolled to produce a rolled Mg alloy material having desired mechanical properties. In the rolling, preheating may be performed in order to enhance plastic workability (rolling property) of the Mg alloy material and to prevent the rolled state from varying in the width direction. For the preheating, for example, heating means such as a heat box 2 illustrated in FIG. 1(B) may be used. In this case, a long Mg alloy material 1 can be heated at one time, which is good in terms of operation efficiency. The heat box 2 is an atmosphere furnace, which is an airtight container that can house the Mg alloy material sheet 1 wound in the form of a coil and in which hot air at a predetermined temperature is supplied and circulated in the container so that the inside of the container can be maintained at a desired temperature. The Mg alloy material sheet 1 may be taken from the heat box 2 without undergoing further treatment, and rolled. With this structure, in particular, it is possible to reduce the time until the heated Mg alloy material sheet 1 contacts the reduction rolls 3, thereby effectively suppressing a decrease in the temperature of the Mg alloy material sheet 1 that occurs until the Mg alloy material sheet 1 contacts the reduction rolls 3. Specifically, for example, the heat box 2 can house the Mg alloy material sheet 1 wound in the form of a coil, and rotatably support the reel 10 that can unwind and take up the Mg alloy material sheet 1. The Mg alloy material sheet 1 is housed in this heat box 2, and is heated to a particular temperature. FIG. 1(B) illustrates a state where a Mg alloy material sheet 1 wound in the form of a coil is housed in a heat box 2. Although the heat box 2 is used in a closed state in reality, for the sake of ease of understanding, the FIGURE illustrates a state where a front face is opened.

In the preheating step, heating is conducted so that the temperature of the Mg alloy material is 300° C. or lower. The preset temperature of the heating means such as a heat box can be selected from a range of 300° C. or lower. In particular, the preset temperature is preferably adjusted so that, immediately before the rolling, a surface temperature of the material is in the range of 150° C. to 280° C. through the all passes. When a Mg alloy material is rolled in a plurality of passes, the temperature of the Mg alloy material tends to be increased by heat generated by working. On the other hand, the temperature of the Mg alloy material may decrease until the Mg alloy material is unwound and contacts the reduction rolls. Accordingly, the present temperature of the heating means is preferably adjusted in consideration of the rolling speed (mainly, the traveling speed of the material during rolling), the distance from the heating means to the reduction rolls, the temperature of the reduction rolls, the number of passes, etc. The preset temperature of the heating means is preferably 150° C. to 280° C., in particular, 200° C. or higher, and particularly preferably 230° C. to 280° C. The heating time may be determined as a time until the Mg alloy material can be heated to a predetermined temperature. However, for a Mg alloy coil material wound in the form of a coil, it is preferable to ensure a sufficient time so that a temperature variation between the inner region and the outer region of the coil is reduced and the temperature of the whole Mg alloy coil material becomes uniform. Furthermore, the heating time may be appropriately determined in consideration of the weight, the dimensions (width and thickness), the number of windings, etc. of the coil.

The material sheet 1 is preferably covered with the protective cover 5 composed of a heat insulating material so that a surface temperature of the Mg alloy material sheet 1 that is preheated and unwound does not vary in the width direction before the Mg alloy material sheet 1 passes through the reduction rolls 3. In particular, the heated state of two edge portions in the width direction of the material sheet 1 is difficult to be maintained, and the two edge portions easily cool. Therefore, it is preferable to cover at least the two edge portions so that the temperature in the width direction does not vary. With this structure, the subsequent rolling can be easily uniformly performed in the width direction, and the rolled state does not tend to vary.

A surface temperature of the Mg alloy material sheet 1 is measured before and after the Mg alloy material sheet 1 passes through the reduction rolls 3. Temperature sensors used therefor are arranged between the reel 10 a and the reduction rolls 3, and between the reel 10 b and the reduction rolls 3. For example, in FIG. 1(A), when a direction in which the material sheet 1 moves from the left side to the right side of the drawing is assumed to be an outward direction, the temperature sensor 4 bf arranged on the left side of the reduction rolls 3 detects the surface temperature of the Mg alloy material sheet 1 immediately before the Mg alloy material sheet 1 passes through the reduction rolls 3, and the temperature sensor 4 bb arranged on the right side of the reduction rolls 3 detects the surface temperature of the rolled sheet immediately after the sheet passes through the reduction rolls 3. On the other hand, when a direction in which the material sheet 1 moves from the right side to the left side of the drawing is assumed to be a return direction, the temperature sensor 4 bf arranged on the right side of the reduction rolls 3 detects the surface temperature of the Mg alloy material sheet 1 immediately before the Mg alloy material sheet 1 passes through the reduction rolls 3, and the temperature sensor 4 bb arranged on the left side of the reduction rolls 3 detects the surface temperature of the rolled sheet immediately after the sheet passes through the reduction rolls 3.

A surface temperature of the Mg alloy material sheet 1 preheated to the above temperature range is measured by the temperature sensor 4 bf before rolling. The temperature sensor 4 bf may be a contact-type sensor that is brought into contact with the material sheet 1 to measure the temperature. The temperature sensor 4 bf is preferably a non-contact-type sensor so as to prevent the material sheet 1 from being damaged. The number and the positions of the temperature sensors 4 bf arranged are appropriately selected so that the temperatures of at least three positions including the central portion and the two edge portions in the width direction of the material sheet 1 can be separately measured. For example, three temperature sensors 4 bf may be arranged above the central portion and the two edge portions to measure the temperatures of each of these portions. As described below, in the case where the difference in temperature at every 100 mm interval in the width direction of the material sheet 1 (rolled sheet) is controlled, temperature sensors, the number of which is determined in accordance with the width of the sheet, are arranged every 100 mm. It is preferable to perform control, for example, change the heating temperature of the preheating or change the heating temperature of auxiliary heating means such as a heat-generating lamp described below on the basis of the temperatures measured by the sensors 4 bf. Thus, the difference in temperature over the width direction of the Mg alloy material sheet 1 is easily reduced.

It is preferable to arrange auxiliary heating means (not illustrated) for reheating the Mg alloy material sheet 1 on the basis of the temperatures measured by the temperature sensors 4 bf. An example of the auxiliary heating means is a heat-generating lamp. The auxiliary heating means is arranged on the reel 10 a (10 b) side with respect to the temperature sensors 4 bf. The number of the auxiliary heating means arranged is appropriately selected so that at least two edge portions in the width direction of the Mg alloy material sheet 1 can be separately heated. In this case, the temperatures of the two edge portions, whose heated state is difficult to be maintained, i.e., which easily cool during rolling, can be separately controlled, and the variation in the temperature in the width direction can be reduced.

By preheating including this reheating, the temperature of the Mg alloy material sheet 1 is preferably controlled so that the difference between the maximum temperature and the minimum temperature of the entire region in the width direction of the Mg alloy material sheet 1 is 8° C. or less, and in particular 5° C. or less within the preset temperature range. In this case, for example, even in the Mg alloy material sheet 1 having a wide width of 1,000 mm or more, the variation in the temperature over the width direction is small, and thus the rolled state of the Mg alloy material sheet 1 does not tend to vary. In addition, the difference in temperature between two points 100 mm away from each other in the width direction of the Mg alloy material sheet 1 is preferably controlled to 6° C. or less, and more preferably 3° C. or less. By reducing the difference in temperature between two close points, the variation in the temperature distribution over the width direction of the Mg alloy material sheet 1 is easily controlled. As a result, the variation in the rolled state of the Mg alloy material sheet 1 can be effectively reduced.

[Rolling]

The Mg alloy material sheet 1 heated by heating means such as the heat box 2 is unwound from the heat box 2, supplied to the reduction rolls 3, and rolled. Specifically, for example, a rolling line illustrated in FIG. 1(A) is constructed. The rolling line includes a pair of reels 10 a and 10 b that can reverse their directions of rotation, and a pair of reduction rolls 3 which are arranged between the pair of reels 10 a and 10 b arranged with a space therebetween, and which are arranged so as to face each other and to sandwich the traveling Mg alloy material sheet 1 therebetween. A coil-shaped Mg alloy material sheet 1 is arranged in a reel 10 a and is unwound, and an end of the Mg alloy material sheet 1 is taken up by the other reel 10 b, whereby the Mg alloy material sheet 1 travels between the reels 10 a and 10 b. During this traveling, the Mg alloy material sheet 1 can be rolled by being sandwiched between the reduction rolls 3. In the example illustrated in FIG. 1(A), the reels 10 a and 10 b are housed in heat boxes 2 a and 2 b, respectively, and the Mg alloy material sheet 1 wound on the reels 10 a and 10 b can be heated by the heat boxes 2 a and 2 b. The heated Mg alloy material sheet 1 is unwound from one of the reels and taken out from one of the heat boxes, travels toward the other heat box, and is taken up by the other reel.

In this embodiment, the two ends of the Mg alloy material sheet 1 are taken up by the reels 10 a and 10 b, and an intermediate region other than the regions taken up by the reels 10 a and 10 b at both ends is introduced into the reduction rolls 3 and subjected to rolling in a plurality of passes. The rolling in each pass is performed by reversing the rotation direction of the reels 10 a and 10 b in every pass. Specifically, reverse rolling is performed. Accordingly, the Mg alloy material sheet 1 is not detached from the reels 10 a and 10 b until the final pass.

In FIG. 1, the number of reduction rolls 3 is illustrative. A plurality of pairs of reduction rolls may be arranged in a direction in which the Mg alloy material sheet 1 travels.

The reduction rolls 3 are heated so that a surface temperature thereof specifically becomes in the range of 230° C. to 290° C. When the surface temperature is 230° C. or higher, the material sheet 1 can be sufficiently maintained in a heated state, and thus the material sheet can be in a state of good plastic workability and rolling can be satisfactorily performed. When the surface temperature is 290° C. or lower, coarsening of the grain size of the material sheet 1 and releasing of work strain introduced by rolling are suppressed, and a rolled sheet having good press workability can be produced.

In the above temperature range, the temperature is controlled so that the difference between the maximum temperature and the minimum temperature in the width direction of a surface of a reduction roll is 10° C. or less. By reducing the difference in temperature over the width direction of the reduction roll 3, the variation in the rolled state in the width direction can be reduced. Specifically, the mechanical properties of the rolled Mg alloy material can be made uniform in the width direction. In addition, the variation in the thickness of the rolled sheet and the occurrence of winding deviation due to this variation in the thickness can be effectively reduced. The difference between the maximum temperature and the minimum temperature in the width direction of the reduction roll 3 is more preferably 5° C. or less.

In addition, the temperature is preferably controlled so that the difference in temperature between two points 100 mm away from each other in the width direction of the reduction rolls 3 is preferably 6° C. or less, and more preferably 3° C. or less. By reducing the difference in temperature between two close points, the variation in the temperature distribution over the width direction of the reduction rolls 3 is easily controlled. As a result, the variation in the rolled state of the Mg alloy material sheet 1 can be effectively reduced. The distance between the two points may be 100 mm or more or 100 mm or less. However, a smaller distance is preferable because the variation in the temperature over the width direction can be more easily controlled.

The temperature of the reduction rolls 3 is checked by the temperature sensor 4 r. The temperature sensor 4 r may also be a contact-type sensor that is brought into contact with a roll 3 to measure the temperature or a non-contact-type sensor. The number and the positions of the temperature sensors 4 r arranged are appropriately selected so that the temperatures of at least three positions including a central portion and two edge portions in the width direction of the roll 3 can be measured. For example, three temperature sensors 4 r may be arranged above the central portion and the two edge portions to measure the temperatures of each of these portions. In the case where the difference in temperature at every 100 mm interval in the width direction of the reduction roll 3 is controlled, temperature sensors, the number of which is determined in accordance with the width of the reduction roll, are arranged every 100 mm.

Furthermore, the temperatures of the material sheet 1 immediately after the material sheet 1 passes through the reduction rolls 3 are also checked by the temperature sensors 4 bb in the same manner. It is preferable to perform temperature control, for example, appropriately change the heating temperature of the reduction rolls 3 on the basis of the temperatures measured by the temperature sensors 4 bb. Thus, the difference in temperature over the width direction of the Mg alloy material sheet 1 is more easily reduced. When the measurement is performed with the temperature sensors 4 bb, the difference in temperature between two points 100 mm away from each other in the width direction of the Mg alloy material sheet 1 is preferably 6° C. or less, and particularly preferably 3° C. or less.

It is believed that the temperature of the whole material sheet 1 wound in the form of a coil does not easily decrease during the transport and installation of the material sheet 1 because the whole material sheet 1 has a heat capacity higher than that of an unwound part of the material sheet 1. In contrast, it is believed that a decrease in the temperature of the material sheet 1 from the time when the material sheet 1 is unwound from a reel 10 or a supply device to the time when the material sheet 1 contacts the reduction rolls 3 is relatively significant. The reason for this is believed to be that such a material sheet 1 is a part of the material as described above and has a low heat capacity, and that the magnesium alloy is a metal having good thermal conductivity and easily cools. The degree of decrease in the temperature of the material sheet 1 until the material sheet 1 contacts the reduction rolls 3 is affected by the thickness of the material sheet 1, the traveling speed of the material sheet 1, etc. The smaller the thickness of the sheet and the lower the rolling speed, the more easily the temperature decreases. It is preferable to supply the material sheet 1 to the reduction rolls 3 before the surface temperature of the material sheet 1 becomes lower than 170° C., preferably at a surface temperature of the material sheet 1 of 180° C. or higher, and particularly preferably 210° C. or higher. The rotation speed (peripheral speed) of the reduction rolls 3 is appropriately adjusted in accordance with the traveling speed of the material. When the rotation speed of the reduction rolls 3 is, for example, 5 to 200 m/min, the rolling can be efficiently performed.

In order to control the temperature of a surface of the reduction rolls 3 as described above, the reduction rolls 3 each have three or more regions in the width direction, and the temperature is controlled in each of the regions. As means for controlling the temperature, for example, a heater such as a cartridge heater may be provided in the reduction rolls 3 (heater method), a liquid such as heated oil (heat transfer oil) may be introduced into the reduction rolls or circulated in the rolls (liquid-circulating method), or a heating fluid whose temperature has been adjusted may be directly allowed to adhere. As specific means for allowing a heating fluid to directly adhering to the reduction rolls 3, for example, gas such as hot air may be blown (hot air method) or a lubricant or the like described below may be applied. Among these methods, in particular, when the reduction rolls 3 are heated by circulating heated oil inside the reduction rolls 3, the reduction rolls 3 can be uniformly filled with the heated liquid in the width direction and the circumferential direction. Thus, the temperature can be rapidly controlled to a predetermined temperature from the inside of the reduction rolls 3 in each of the regions, and the difference between the maximum temperature and the minimum temperature in the width direction of the rolls can be easily reduced to the above range. The temperature of the liquid circulated is preferably the preset surface temperature of the reduction rolls 3 plus about 10° C., though it depends on the dimensions (width and diameter) and the material of the reduction rolls 3, and the widths and the positions of the regions. For example, a liquid circulation mechanism used in a water-cooled copper or the like can be applied to the circulation of the liquid. In the heater method, a plurality of heaters are preferably adjusted and housed in each of the regions in order to reduce the variation in the temperature in the width direction of the reduction rolls 3. Specifically, it is preferable to change the number of heaters or to change the temperatures of the heaters in the central portion of the roll where the heated state is easily maintained and in the edge portions of the roll where the heated state is difficult to be maintained. A sliding contact may be used for electrical connection between each heater side and the power supply side in the rotation axis of each of the reduction rolls 3. In the hot air method, the temperature of the gas, the amount of blowing, the number of gas outlets, the arrangement positions of the gas outlets, etc. may be adjusted.

In the rolling of each pass, the rolling reduction per pass can be appropriately selected. The rolling reduction per pass is preferably 10% or more and 40% or less, and the total rolling reduction is preferably 75% or more and 85% or less. By rolling a material a plurality of times (in a plurality of passes) with rolls at such a rolling reduction, a desired thickness of the resulting rolled sheet can be obtained, the average grain size can be reduced, press workability can be enhanced, and the generation of defects such as surface cracks can be suppressed.

In the rolling, a lubricant is preferably used because friction between the material and the reduction rolls can be reduced, and the rolling can be satisfactorily performed. The lubricant may be applied onto the reduction rolls as required. However, it was found that, for some types of lubricants, a lubricant remaining on the material is burned by heat in the subsequent preheating step or by heat due to contact with the reduction rolls, and an affected layer is formed. It was also found that, when such an affected layer is present, the material is not uniformly rolled, the thickness of the rolled sheet may vary, and the material may meander or travel in an inclined manner in one direction (transversely moves) because of this variation in the thickness, which may easily cause significant winding deviation. Furthermore, it was also found that the lubricant tends to remain on the two edge portions rather than the central portion in the width direction of the material, though details of the mechanism responsible for this are not clear. Therefore, it is preferable to use a lubricant that does not form an affected layer at 290° C., which is the maximum of the heating temperature of the reduction rolls, and in consideration of a margin, about 300° C. In order to prevent a lubricant or an affected layer from being locally present on the material as described above, the lubricant on the surface of the material is preferably leveled immediately before the material is supplied to the reduction rolls. For example, leveling means such as a brush or a wiper may be arranged on the upstream side of the reduction rolls so as to level unevenness of the lubricant on the surface of the material.

In order to adjust the tension applied to the material sheet 1 during rolling, pinch rollers (not illustrated) may be arranged at the upstream side and the downstream side of the reduction rolls 3. In order to prevent a decrease in the temperature of the material due to contact with the pinch rolls, the pinch rolls are preferably heated to about 200° C. to 250° C.

(Winding)

The rolled sheet obtained after rolling is wound in the form of a coil. A series of steps including the preheating step, the rolling step, and this winding step are continuously repeatedly performed, thus conducting rolling with rolls a desired number of times. The resulting rolled sheet (magnesium alloy sheet) is then finally wound in the form of a coil. The magnesium alloy sheet constituting the resulting coil material has a structure including work strain (shear band) introduced by rolling. Since the magnesium alloy sheet has such a structure, dynamic recrystallization occurs in the magnesium alloy sheet during plastic working such as press working and thus the magnesium alloy sheet has good plastic workability. In particular, in the rolling of the final pass, when the rolled sheet is wound while the temperature of the rolled sheet immediately before winding is controlled to a temperature at which recrystallization does not occur, specifically, a temperature of 150° C. or lower, a magnesium alloy sheet having good flatness can be obtained and the magnesium alloy sheet can have a structure in which the work strain sufficiently remains. In order to control the temperature of the rolled sheet immediately before winding to a temperature at which recrystallization does not occur, the traveling speed of the material may be adjusted. Alternatively, the rolled sheet may be cooled by forced cooling such as air blast. In this case, the temperature can be adjusted to a predetermined temperature within a short time, which is good in terms of operation efficiency.

(Straightening Step)

The wound coil material can be used as a product (typically, a raw material of a magnesium alloy material, such as a plastic working material) without undergoing further treatment. Furthermore, this coil material may be unwound, a predetermined bending may be provided to the rolled sheet, and thus straightening of work strain introduced by rolling may be performed. A roller leveler can be suitably used in the straightening. The roller leveler includes at least one pair of rollers facing each other, and provides bending by allowing a material to insert between the rollers. In particular, a roller leveler that can be suitably used is one that includes a plurality of rollers arranged in a zigzag manner and that can repeatedly provide bending to a rolled sheet by allowing the rolled sheet to pass between the rollers. By conducting such straightening, a magnesium alloy sheet having excellent flatness can be produced. In addition, since the work strain is sufficiently present, the magnesium alloy sheet can have good plastic workability such as press workability. Warm straightening may be performed in which bending is provided to a rolled sheet using heated rollers including heating means such as a heater. In this case, breaking etc. are not readily generated. The temperature of the rollers is preferably 100° C. or higher and 300° C. or lower. The amount of bending provided by the straightening can be adjusted by adjusting the size and the number of rollers, the distance (gap) between rollers arranged so as to face each other, the distance between rollers that are adjacent in a direction in which the material travels, and the like. The magnesium alloy sheet (rolled sheet) serving as a material may be heated in advance before the straightening is performed. A specific heating temperature is 100° C. or higher and 250° C. or lower, and preferably 200° C. or higher.

The magnesium alloy sheet subjected to the straightening step can be used as a product (typically, a raw material of a magnesium alloy material, such as a plastic working material) without undergoing further treatment. In order to further improve the surface state, surface polishing may be performed by using a polishing belt or the like.

<Operations and Advantages>

According to the rolled Mg alloy material and the method for producing a rolled Mg alloy material according to the above embodiments, the following advantages are achieved.

(1) A rolled Mg alloy material has a wide width of 1,000 mm or more, and mechanical properties thereof are substantially uniform in the width direction. Accordingly, when plastic working is performed on this rolled material, substantially uniform working can be performed at any position.

(2) Even when a single rolled material is divided in the width direction to prepare a plurality of rolled Mg alloy sheets each having a narrow width, since the rolled material has mechanical properties that are uniform in the width direction, rolled materials having substantially the same mechanical properties can be obtained.

(3) According to the production method described above, by reducing the difference in temperature over the width direction of reduction rolls, the variation in the rolled state in the width direction can be reduced even when the width is 1,000 mm or more. Therefore, it is possible to produce a rolled Mg alloy material which has a width of 1,000 mm or more and whose mechanical properties are uniform in the width direction.

Test Examples

As test examples, the following rolled Mg alloy materials are prepared and mechanical properties thereof are examined. First, a Mg alloy material sheet having a composition corresponding to AZ91 containing Mg-9.0 mass % Al-1.0 mass % Zn, and a Mg alloy material sheet having a composition corresponding to AZ31 containing Mg-3.0 mass % Al-1.0 mass % Zn are produced by twin-roll casting. These material sheets each have a thickness of 5.0 mm, a width of 1,020 mm, and a length of 1,000 mm. A solution treatment is performed at 400° C. for 20 hours on each of the samples prior to rolling. Subsequently, rolling is performed under the conditions described below. Thus, samples 1 to 3 composed of AZ91 and samples 4 to 6 composed of AZ31 were prepared.

(Rolling Conditions)

Rolling in a plurality of passes, rolling reduction: 15% to 25%/pass

Final thickness: Rolling was performed until the thickness became 1.0 mm (width: 1,020 mm), total rolling reduction: 80%

Preheating of material sheet (in heating furnace, heating time: 30 min)

Method for heating reduction rolls: Heated from the outside of the rolls

Reduction rolls were heated by the following method. The reduction rolls were each equally divided into three regions in the width direction thereof, and a lubricant whose temperature had been adjusted was directly applied onto the three regions. In sample 1, the lubricant whose temperature had been adjusted to 250° C. to 255° C. was applied onto the center of the three regions, and the lubricant whose temperature had been adjusted to 255° C. to 260° C. was applied onto both sides of the center so that a roll surface temperature became uniform in the width direction. On the other hand, in sample 4, the lubricant whose temperature had been adjusted to 230° C. to 235° C. was applied onto the center, and the lubricant whose temperature had been adjusted to 235° C. to 240° C. was applied onto both sides of the center so that a roll surface temperature became uniform in the width direction.

In performing rolling, the temperature of a surface of the Mg alloy material sheet immediately before rolling, the temperature of a surface of the reduction roll, and the temperature of a surface of the rolled Mg alloy sheet immediately after rolling were measured and determined as follows. In a region on the surface of the reduction roll that the material sheet contacts, an arbitrary straight line is set along a width direction (direction parallel to the axial direction) of the roll, and the temperature is measured at a plurality of points along the straight line. In this example, the arbitrary straight line was set on each of the surface of the Mg alloy material sheet, the surface of the reduction roll, and the surface of the rolled Mg alloy material. Along the straight line, a total of 11 points including a point 10 mm from an edge in the width direction and 10 points spaced away from the point at 100 mm intervals were determined, and the temperatures of the respective points were measured by non-contact type temperature sensors. In this measurement, the temperatures in the width direction of the reduction roll are measured at positions on the surface of the reduction roll, the positions being shifted from a region where the lubricant is sprayed, so as not to measure the temperature of the lubricant. The values are shown in Tables I to III.

TABLE I Upper row: surface temperature of Mg alloy material sheet (° C.) Maximum Lower row: difference in temperature between two points (° C.) temperature- Sample Measurement point (mm) minimum No. 10 110 210 310 410 510 610 710 810 910 1010 temperature 1 251 255 257 255 257 256 258 256 255 253 250 8 4 2 2 2 1 2 2 1 2 3 2 243 249 257 256 258 260 263 259 258 250 236 27 6 8 1 2 2 3 4 1 8 14  3 281 275 259 255 252 250 251 256 259 272 288 38 4 16  4 3 2 1 5 3 13 16  4 225 230 232 230 231 232 230 231 232 228 224 8 5 2 2 1 1 2 1 1 4 4 5 212 220 231 231 232 231 231 229 224 215 210 22 8 11  0 1 1 0 2 5 9 5 6 245 239 232 229 230 229 231 229 232 239 243 16 6 7 3 1 1 2 2 3 7 4

TABLE II Upper row: surface temperature of reduction roll (° C.) Maximum Lower row: difference in temperature between two points (° C.) temperature- Sample Measurement point (mm) minimum No. 10 110 210 310 410 510 610 710 810 910 1010 temperature 1 249 250 252 250 251 253 252 250 251 250 248 5 1 2 2 1 2 1 2 1 1 2 2 241 248 247 252 256 256 255 253 248 243 238 18 7 1 5 4 0 1 2 5 5 5 3 250 249 251 252 250 253 251 251 250 251 250 4 1 2 1 2 3 2 0 1 1 1 4 228 229 232 231 231 232 231 230 230 230 229 4 1 3 1 0 1 1 1 0 0 1 5 220 229 226 231 234 235 234 233 229 223 218 17 9 3 5 3 1 1 1 4 6 5 6 229 229 232 232 231 233 232 231 232 231 230 4 0 3 0 1 2 1 1 1 1 1

TABLE III Upper row: surface temperature of rolled Mg alloy sheet (° C.) Maximum Lower row: difference in temperature between two points (° C.) temperature- Sample Measurement point (mm) minimum No. 10 110 210 310 410 510 610 710 810 910 1010 temperature 1 250 254 255 256 254 255 257 255 253 252 249 8 4 1 1 2 1 2 2 2 1 3 2 242 247 251 254 257 258 258 253 256 248 235 23 5 4 3 3 1 0 5 3 8 13 3 260 267 255 254 252 253 252 254 257 262 270 18 7 12 1 2 1 1 2 3 5 8 4 226 230 233 231 231 231 230 231 232 229 226 7 4 3 2 0 0 1 1 1 3 3 5 215 225 229 232 234 233 234 231 223 218 212 22 10 4 3 2 1 1 3 8 5 6 6 241 237 231 230 230 232 232 230 233 237 240 11 4 6 1 0 2 0 2 3 4 3

[Evaluation of Mechanical Properties]

For samples 1 to 6 composed of the resulting rolled Mg alloy materials, the following properties were evaluated.

[Basal Plane Peak Ratio]

A basal plane peak ratio of each of samples 1 to 6 was measured on the basis of X-ray diffraction peak intensities. In this measurement, X-ray diffractometry was conducted at positions 50 mm (edge portion), 500 mm (central portion), and 950 mm (edge portion) from an edge in the width direction on a surface of each sample to determine the peak intensities of the (002) plane, the (100) plane, the (101) plane, the (102) plane, the (110) plane, and the (103) plane. A basal plane peak ratio O_(E) of the edge portion and a basal plane peak ratio O_(C) of the central portion were determined from the results, and a ratio O_(E)/O_(C) was also determined. The basal plane peak ratios O_(C) and O_(E) are represented by the following formulae:

Basal plane peak ratio O_(C): I_(C)(002)/{I_(C)(100)+I_(C)(002)+I_(C)(101)+I_(C)(102)+I_(C)(110)+I_(C)(103)}

Basal plane peak ratio O_(E): I_(E)(002)/{I_(E)(100)+I_(E)(002)+I_(E)(101)+I_(E)(102)+I_(E)(110)+I_(E)(103)}

In the above formulae, I_(C)(002), I_(C)(100), I_(C)(1101), I_(C)(102), I_(C)(110), and I_(C)(103) represent X-ray diffraction peak intensities of the above respective planes in the central portion, and I_(E)(002), I_(E)(100), I_(E)(101), I_(E)(102), I_(E)(110), and I_(E)(103) represent X-ray diffraction peak intensities of the above respective planes in the edge portion.

The results are shown in Table IV.

[Average Grain Size]

An average grain size of each of samples 1 to 6 was measured in accordance with “Steels-Micrographic determination of the grain size JIS G 0551 (2005)”. This measurement was conducted at positions 50 mm (edge portion), 510 mm (central portion), and 970 mm (edge portion) from an edge in the width direction of a cross section orthogonal to the rolling direction of each sample. An average grain size ratio D_(E)/D_(C) of the average grain size of the edge portion to the average grain size of the central portion was determined from the results. The results are shown in Table IV.

[Tensile Test]

An elongation, a tensile strength, and a 0.2% proof stress of each of samples 1 to 6 were measured in accordance with “Method of tensile test for metallic materials JIS Z 2241 (1998)”. In this measurement, at positions 50 mm (edge portion), 510 mm (central portion), and 970 mm (edge portion) from an edge in the width direction of each sample, a JIS No. 13B specimen (JIS Z 2201 (1998)) was cut so that the longitudinal direction of the specimen corresponded to the rolling direction, and the tensile test was performed using the specimen. An elongation ratio E_(E)/E_(C), a tensile strength ratio Ts_(E)/Ts_(C), and a 0.2% proof stress ratio Ps_(E)/Ps_(C) of the edge portion to the central portion were respectively determined from the results.

The results are summarized and shown in Table V.

TABLE IV Basal plane peak ratio Ratio of basal Grain size Measurement plane peak Measurement Grain size ratio Sample point (mm) ratio (OE/OC) point (mm) (DE/DC) No. 50 500 950 50/500 950/500 50 510 970 50/510 970/510 1 0.862 0.860 0.861 1.002 1.001 5.2 5.3 5.2 0.981 0.981 2 0.981 0.847 0.988 1.158 1.166 4.3 6.5 4.4 0.662 0.677 3 0.760 0.849 0.730 0.895 0.860 8.9 5.5 9.5 1.618 1.727 4 0.711 0.710 0.711 1.001 1.001 4.9 5.0 4.9 0.980 0.980 5 0.825 0.697 0.830 1.184 1.191 3.9 5.6 4 0.696 0.714 6 0.610 0.704 0.597 0.866 0.848 9.5 5.6 10.1 1.696 1.804

TABLE V 0.2% Proof stress Tensile strength (MPa) 0.2% proof (MPa) Tensile strength Elongation (%) Measurement stress ratio Measurement ratio Measurement Elongation ratio Sample point (mm) (PsE/PsC) point (mm) (TsE/TsC) point (mm) (EE/EC) No. 50 510 970 50/510 970/510 50 510 970 50/510 970/510 50 510 970 50/510 970/510 1 267 269 265 0.993 0.985 332 335 330 0.991 0.985 10 12 11 0.833 0.917 2 269 254 280 1.059 1.102 335 321 354 1.044 1.103 10 13 8 0.769 0.615 3 239 265 238 0.902 0.898 313 349 312 0.897 0.894 10 12 10 0.833 0.833 4 237 238 236 0.996 0.992 303 305 301 0.993 0.987 18 18 17 1.000 0.944 5 238 225 248 1.058 1.102 308 292 322 1.055 1.103 11 18 14 0.611 0.778 6 209 234 210 0.893 0.897 279 309 275 0.903 0.890 17 20 16 0.850 0.800

[Results]

The above results showed that, when a Mg alloy material having a wide width of 1,000 mm or more is rolled, the variation in the rolled state in the width direction can be reduced by reducing the difference in temperature over the width direction of the surface of a reduction roll. In addition, it was also found that the variation in the rolled state can be further reduced by reducing the difference in temperature over the width direction of the surface of the Mg alloy material before rolling, and it is possible to perform rolling that is uniform in the width direction of the Mg alloy material. It was also found that a rolled Mg alloy material whose mechanical properties are uniform in the width direction is obtained by reducing the variation in the rolled state.

It is to be understood that the embodiments described above can be appropriately changed without departing from the gist of the present invention, and are not limited to the configurations described above.

INDUSTRIAL APPLICABILITY

The rolled Mg alloy material of the present invention can be suitably used as a material of component members of various electric/electronic devices, in particular, housings of mobile or compact electric/electronic devices, and a material of members which are used in various fields and for which a high strength is desired, for example, component members of transport machines such as automobiles and aircraft. The method for producing a rolled Mg alloy material of the present invention can be suitably employed in the production of a rolled Mg alloy material which has a width of 1,000 mm or more and whose mechanical properties are uniform in the width direction.

REFERENCE SIGNS LIST

-   -   1 Mg alloy material sheet     -   2, 2 a, 2 b heat box     -   3 reduction roll     -   4 bf, 4 bb, 4 r temperature sensor     -   5 protective cover     -   10, 10 a, 10 b reel 

What is claimed is: 1.-13. (canceled)
 14. A rolled magnesium alloy material produced by rolling a magnesium alloy material with a reduction roll, wherein the rolled magnesium alloy material has a width of 1,000 mm or more, and in a width direction of the rolled magnesium alloy material, a ratio O_(E)/O_(C) of a basal plane peak ratio of an edge portion to a basal plane peak ratio of a central portion satisfies 0.89≦O_(E)/O_(C)≦1.15, where the basal plane peak ratio O_(C) of the central portion and the basal plane peak ratio O_(E) of the edge portion are respectively represented by formulae below: basal plane peak ratio O_(C): I_(C)(002)/{I_(C)(100)+I_(C)(002)+I_(C)(101)+I_(C)(102)+I_(C)(110)+I_(C)(103)} basal plane peak ratio O_(E): I_(E)(002)/{I_(E)(100)+I_(E)(002)+I_(E)(101)+I_(E)(102)+I_(E)(110)+I_(E)(103)} where I_(C)(002), I_(C)(100), I_(C)(101), I_(C)(102), I_(C)(1110), and I_(C)(103) respectively represent X-ray diffraction peak intensities of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane, and a (103) plane in the central portion, and I_(E)(002), I_(E)(100), I_(E)(101), I_(E)(102), I_(E)(110), and I_(E)(103) respectively represent X-ray diffraction peak intensities of the (002) plane, the (100) plane, the (101) plane, the (102) plane, the (110) plane, and the (103) plane in the edge portion.
 15. The rolled magnesium alloy material according to claim 14, wherein an average grain size ratio D_(E)/D_(C) of the edge portion to the central portion satisfies 0.7≦D_(E)/D_(C)≦1.5, where D_(C) denotes an average grain size of the central portion of a cross section orthogonal to a rolling direction and D_(E) denotes an average grain size of the edge portion of a cross section orthogonal to a rolling direction.
 16. The rolled magnesium alloy material according to claim 14, wherein an elongation ratio E_(E)/E_(C) of the edge portion to the central portion satisfies 2/3≦E_(E)/E_(C)≦3/2, where E_(C) denotes an elongation of the central portion in a tensile test in a rolling direction and E_(E) denotes an elongation of the edge portion in a tensile test in a rolling direction.
 17. The rolled magnesium alloy material according to claim 14, wherein a tensile strength ratio Ts_(E)/Ts_(C) of the edge portion to the central portion satisfies 0.9≦Ts_(E)/Ts_(C)≦1.1, where Ts_(C) denotes a tensile strength of the central portion in a tensile test in a rolling direction and Ts_(E) denotes a tensile strength of the edge portion in a tensile test in a rolling direction.
 18. The rolled magnesium alloy material according to claim 14, wherein a 0.2% proof stress ratio Ps_(E)/Ps_(C) of the edge portion to the central portion satisfies 0.9≦Ps_(E)/Ps_(C)≦1.1, where Ps_(C) denotes a 0.2% proof stress of the central portion in a tensile test in a rolling direction and Ps_(E) denotes a 0.2% proof stress of the edge portion in a tensile test in a rolling direction.
 19. The rolled magnesium alloy material according to claim 14, wherein the magnesium alloy material contains aluminum in an amount of 5% by mass or more and 12% by mass or less.
 20. A magnesium alloy structural member produced by plastically working the rolled magnesium alloy material according to claim
 14. 21. A method for producing a rolled magnesium alloy material comprising rolling a magnesium alloy material with a reduction roll to produce a rolled magnesium alloy material, wherein the magnesium alloy material has a width of 1,000 mm or more, the reduction roll has three or more regions in a width direction, and the temperature is controlled in each of the regions so that a difference between a maximum temperature and a minimum temperature is 10° C. or less in the width direction of a surface of the reduction roll.
 22. The method for producing a rolled magnesium alloy material according to claim 21, wherein the temperature is controlled by introducing, into the reduction roll, heat transfer oil whose temperature has been adjusted.
 23. The method for producing a rolled magnesium alloy material according to claim 21, wherein the temperature is controlled by allowing a heating fluid whose temperature has been adjusted to adhere to the surface of the reduction roll.
 24. The method for producing a rolled magnesium alloy material according to claim 21, wherein the temperature is controlled so that, on the surface of the reduction roll, a difference in temperature between two points 100 mm away from each other in the width direction is 6° C. or less.
 25. The method for producing a rolled magnesium alloy material according to claim 21, wherein preheating is performed so that, on a surface of the magnesium alloy material immediately before the magnesium alloy material passes through the reduction roll, a difference between a maximum temperature and a minimum temperature in the width direction is 8° C. or less.
 26. The method for producing a rolled magnesium alloy material according to claim 21, wherein preheating is performed so that, on a surface of the magnesium alloy material immediately before the magnesium alloy material passes through the reduction roll, a difference in temperature between two points 100 mm away from each other in the width direction is 6° C. or less, and the temperature is controlled so that, on the surface of the rolled magnesium alloy material immediately after the magnesium alloy material passes through the reduction roll, a difference in temperature between two points 100 mm away from each other in the width direction is 6° C. or less. 